1.1. Technical Field
The present invention relates to a wire drawing die made of synthetic single crystal diamond which is used for various purposes such as drawing copper wire for winding, steel cord, stainless steel wire, welding wire, and the like; and it relates to a method of producing wire drawing dies by cleaving diamond.
1.2. Background Art
Diamond is a material having the greatest hardness and highest modulus of elasticity of all known materials. Furthermore, extremely pure diamond has additionally the highest thermal conductivity and the highest transmittance in the infrared spectrum. Thus, diamond is a material for which there are no comparable substitutes.
There are two general methods for synthesizing diamond using high temperature and high pressure. In one method, carbon material to be converted to diamond is mixed or brought into contact with a solvent metal such as iron, cobalt or nickel. A stable high pressure and temperature whereby the carbon is converted into diamond under the action of the solvent metal is used. According to such a method, the solvent metal penetrates into the carbon material, whereby the carbon is caused to diffuse through the solvent metal which is in the form of a thin film, to generate diamond. According to this method, diamond is spontaneously nucleated, and rapidly grows until it reaches a certain size. Considerable quantities of fine diamond powder have been synthesized by this method to be applied to, e.g. abrasives. However, large diamond crystals of high quality cannot be synthesized by the aforementioned method.
On the other hand, a method of synthesizing large diamond crystals of good quality is disclosed in U.S. Pat. No. 3,297,407 issued Jan. 10, 1967 to R. H. Wentorf, Jr. Furthermore, U.S. Pat. No. 4,632,817 issued Dec. 30, 1986 Yazu, provides a method for synthesizing a number of large diamond crystals of high quality simultaneously from a plurality of seed crystals. This method is generally called a temperature gradient method.
The present invention relates to using a diamond synthesized by the temperature gradient method.
Most diamond crystal dies are made of natural diamond, whose (110) or (111) faces are polished and then at least one face vertical to said faces is also polished for observation of holing. After the preparations, a wire drawing hole which is vertical to (110) or (111) faces is formed by a laser beam, electric discharge or ultrasonic grinding, while observing through the polished face. Synthetic diamond crystals have been marketed and various attempts to use them for wire drawing have been carried out.
However, the tool life using the prior art displayed a wide range and it causes a lack of reliability. Especially, the tool life made of natural single crystal diamonds can vary widely for the following reasons.
First, the faults of the natural diamond crystal wire drawing dies are the following: It is said that the die having a wire drawing hole vertical to the (110) face, has generally the highest wear resistance and the longest life as shown on Junkatsu (Lubrication), vol. 112, No. 11, 1967. However, the tool life of the dies have such a range according to following reasons.
Second, it is difficult to judge precisely where the (110) face is because natural diamonds have been rounded, and have variety of shapes depending upon the degree of dissolution. As a result, it requires considerable skill to determine the (110) faces of a natural diamond used as wire drawing dies. Therefore, most of the wire drawing hole directions are not intended, and it makes the tool life short.
Third, according to the natural diamond, they often have a soft portion and hard portion on one single crystal; the wire drawing hole has partial wear during drawing and such wire drawing holes are deformed varying from complete circles to distorted ones. This is the cause of a shortened tool life.
Recently, high quality synthetic diamond has been obtained by the temperature gradient method and is on the market. In spite of the many attempts to use the synthetic diamond for a wire drawing die, a high quality wire drawing die has not been obtained yet and the reasons are the following:
First, synthesized or slightly polished idiomorphic diamond is generally used for the wire drawing die. In order to hold the diamonds to the tool holders, hexahedron diamond is suitable. This is mainly covered with (100) faces, and the wire drawing holes are vertical to the (100) faces. However, a drawing hole of such direction has inferior wear resistance and a short tool life.
Second, some of the synthetic diamonds don't have uniform concentration of nitrogen. This causes partial wear of the drawing hole.
Third, when the diamond is held to a tool holder using the surfaces of grown diamond or polished surfaces, it requires a large stress to hold the diamond in place, because the friction coefficient between the diamond surfaces and holder is very low. Therefore, the wire drawing die often cracks during drawing.
Concerning the dividing diamond method, cutting and cleavage are well known. In the former method, there are two ways; one of which is represented by the following. A cutting blade in which diamond particles are embedded rotates at high speed, presses forward on the diamond and performs the cutting. The other is thermal cutting, for example, using a laser beam.
Concerning the latter method, a diamond is grooved by the other diamond or a cutting blade and then a knife-shaped wedge is struck in the groove and a diamond is cleaved, as shown on pages 411-412 of Kesshokogaku Handbook. The latter is thought to be the best method to divide a diamond economically, taking advantage of diamond's propensity to be easily cleaved parallel to the (111) faces. The method does not require a width loss equal to the cutting blade, especially when compared to the former method and also does not require a great deal of time to divide.
However, only commercial diamond cutters use this method to divide large diamonds for jewelry and ornamental applications. According to this method, it requires a high level of skill, and the possibility to destroy a high priced diamond is great, and also makes the probability of an uneven divided plane high, as shown in Diamond (Sanyo Shuppan Bouekiu Ltd.), pages 216-218 and "Ceramics" 11 (1976) No. 6 page 520, the method is not used in industrial fields. In addition, it is impossible to divide a diamond to get plural, thin and uniform plates which are indispensable for heat sink and wire drawing dies.
The subjects of the prior art have been made clear by the present inventors' research. They are: The angle between the groove and the cleavage plane (111) faces must be at most 0.5.degree. to obtain an even cleavage plane. However, it is impossible to groove such accuracy using another diamond or a cutting blade because a diamond is hard enough to get such an accuracy of 0.5". Also, the lineability of the groove has to be within 0.1 mm to cleave, but it is very difficult to groove-in such accuracy using the prior arts.